It looks like the stock 3.73-to-P235/75 drive/tire ratio give the 4.0's the best mileage from what I've seen across the forum. I have 4.10's and 33's, which is almost the exact same ratio as stock, and JD has what 4.88's and 36's I think.. which is also dang close to the stock ratio.
On the last wheeling trip we did, JD, me and a 3rd Gen owner Will drove about an hour to the site, played around in the mud for about an hour or so and then drove about 45 minutes the next day to meet the rest of the group before filling up. Will's 3rd gen with 3.73's used about 9.5 gallons, I used 8.5 and JD (who drove from VA beach to Richmond before all that) used like what 13-15? I think we all figured we got about 15.5 MPG's, half of it being in 4x4 and in the mud.